Regulation of adipocyte cyclic nucleotide metabolism by lipin 1

Obesity represents a disruption of energy balance in which in energy input exceeds output, and most of the excess calories consumed are converted to triglycerides and stored in adipose tissue. Current clinical approaches for the treatment of obesity and its related complications presently focus on modifications of diet and exercise. These strategies have proved to be of limited effectiveness and the prevalence of obesity continues to increase. Although efforts to develop pharmacological therapies for treatment of obesity focusing on appetite suppression and interference with lipid absorption have met with moderate success, recent advances using mouse models suggest that targeting enzymes and pathways that dictate the balance between triglyceride storage and metabolism may be an effective strategy. Lipin1 is a master regulator of adipocyte differentiation and metabolism. These functions are primarily mediated by dephosphorylation of phosphatidic acid (PA) to form diacylglycerol (DG) which is an obligatory intermediate in triglyceride synthesis. Lipin1 also functions as a nuclear transcriptional co-activator that regulates expression of genes necessary for adipocyte differentiation and function. Lipin1 could therefore be an attractive target for therapies that target triglyceride accumulation and increased adiposity in obese individuals. Because the transcriptional co activator function of Lipin 1 is necessary for adipocyte differentiation, Lipin1 deficient mice carrying a spontaneous inactivating mutation in the lipin 1 gene fail to develop adipose tissue and instead exhibit a fatty liver phenotype after birth that resolves due to compensatory expression of other lipin isoforms. While these animals have multiple metabolic abnormalities, the physiological function of lipin 1 in adipocytes is not known. To investigate this important issue we are collaborating with Brian Finck to use cre-loxP mediated recombination to inactivate the lipin 1 gene in mice after the adipocyte differentiation program has been initiated. This was accomplished by crossing mice carrying a previously described loxP flanked lipin 1 allele with mice expressing cre recombinase under control of the adiponectin promoter. In the course of characterizing these animals (adn-lipin1 -/- mice) we found that the cre deleted lipin 1 allele expresses a variant lipin 1 protein lacking the N-terminal 115 amino acids. Characterization of this variant protein in our laboratory revealed that it lacks PA phosphatase activity but retains its function as a nuclear transcriptional co-activator. These animals therefore harbor a serendipitously-generated lipin 1 “separation of function” allele providing a unique opportunity to define the physiological function of lipin 1 PA phosphatase activity in differentiated adipocytes. This is a particularly interesting issue because emerging evidence implicates lipin1 in regulation of several other aspects of adiopocyte metabolism including mitochondrial homeostasis and oxidative metabolism and the formation of lipid droplets. In addition to their roles as intermediates in triglyceride and phospholipid metabolism, the lipin 1 substrate and product PA, and DG are well established to function as intracellular messenger molecules that regulate protein kinases and pathways that control membrane dynamics and vesicular transport. One interesting and unexpected phenotype of adn- lipin1 -/- mice is a defect in lipolysis which appears to be due to enhanced adipocyte responsiveness to adrenergic signaling due to increased protein kinase A activity. Strong evidence that the lipin 1 substrate PA is an activator of certain cyclic AMP phosphodiesterase enzymes has been reported. Accordingly we propose to test the hypothesis that lipin 1 is a negative regulator of adipocyte cAMP signaling through degradation of the PDE activator PA. Mutations in lipin 1 are associated with human disease and we have recently contributed to a study identifying functional consequences of alterations in lipin1 mRNA splicing and message stability in obesity. Understanding the regulation and function of this important metabolic regulator is therefore critical to our understanding of human metabolic disease and, as discussed above, the potential development of pharmacological interventions for diet induced obesity. The work proposed here would define a new facet of lipin 1 biology and provide evidence for a previously unappreciated link between phospholipid metabolism and cyclic nucleotide signaling.